Yeah...I've checked everything. It's weird - when I watch HBO Go, my receiver lights up all the 5.1 speaker display lights, but displays stereo and only the front left and right speakers produce sound. All the other apps just say stereo.

Turns out I'm an idiot. Surround settings on my receiver were changed somehow for that input...cycled through and now am back to regular 5.1. Only took me a few months to figure that out.